I'm not sure this is possible.  What I want to do is make the sort of 
assignment we see in /etc/hosts but have it work only within a script.

Here's the problem.  machineA is behind a firewall, but it is accessible 
via ssh from machineB, so I've written a script that lets me to ssh to 
machineA via ssh through machineB using port forwarding:

ssh -f -L 25922:machineA:22 ${USERB}@machineB sleep 1 ; ssh -X -p 25922 ${USERA}@localhost

It would be nice if I could use scp in a fairly straightforward way while 
connected that way.  (Ignore usernames for the rest of this to keep it 
simple.)  For example, I wish this would just work:

scp machineA:file .

That can't work, but can I write a script that would make it work?  That 
script would read that command and execute this one:

scp -P 25922 localhost:file .

So it would be neat if I could write the script so that it could 
automatically convert "machineA" to localhost or 127.0.0.1.  I have this 
line in /etc/hosts:

127.0.0.1	localhost

Is there any way to basically make this assignment inside of the script 
(so that it doesn't change anything except within the script)?:

127.0.0.1	localhost machineA

Then something like this might work in a script:

scp -P 25922 $*

The user could then just do something like this:

scpA.bash machineA:file .

Is there any hope of making that work?  If not, maybe I can search for 
"machineA" and replace with localhost.

Mike